Plural metallic coatings on uranium and method of applying same



2,851,766 Patented Sept. 16, 1958 MEETALLIC COATINGS N AND METHOD OFAPPLYING SAME 8' Claims. (Cl. 29-194) This invention relates to theapplication of metallic coatings to chromium surfaces.

Metallic chromium has many desirable properties as a' structural orcoating metal. In some cases, however, its corroson resistance isinadequate and application of other metals possessing greater resistanceto the specific corrosive elements involved is desirable. In other casesthe chromium exerts undesirable electro-chemical effects andapplicationof coatings to overcome these effects may benecessary.

In the protection of metallic uranium, chromium plate ings have verybeneficial properties for preventing diifusion of metallic uranium.However, for some purposes the metallic uranium is not adequatelyprotected from corrosive influences by the chromium alone. Hence it isdesirable to apply metallic coatings to the chromium platings. Thechromium platings have been found to be particularly refractory towetting by other coating metals. As a consequence only poor, incompletecoatings are obtained when coating of the chromium platingsby'lowmelting hot-dip coating metals is attempted.

An object of the present invention is the application of' adherentmetallic coatings to chromium surfaces. A further object, is theprovision of corrosion-resistant protectilve coatings on chromiumplatings. A still further object is the protection of metallic uraniumby means (if-chromium platings coated with hot-dip coatings. Otherobjects will appear from the following description of the invention.

In accordance with my invention chromium surfaces are coated by applyingcoating metals consisting of or containing aluminum. I have found thatthe aluminum wets the chromium and provides a firm bond between thechromium and the coating metal. The proportion of aluminum necessary toaccomplish the purposes of the invention is not large. Thus zinccoatings containing as little as 1% of aluminum have been appliedsuccessfully. On the other hand coatings containing more than 99% ofaluminum also may be applied. The process is particularly applicable tobright chromium electroplatings on metallic uranium. Most suitableconditions for applying the coatings will vary depending upon thecomposition of the coating metal employed. Thus temperatures from 450 C.or less to 750 C. or more may be used, depending upon the melting pointof the metal to be applied. In the application of zinc coatingscontaining from 1% to 5% of aluminum, temperatures from 150 to 650 C.may be used, the lower temperatures ures between 600 C. and 750 C.

The hot-dip coatings may be applied with or without fiux. Alkali-metalhalide fluxes, especially those containing small proportions ofalkali-metal fluoride, have een found to be particularly satisfactory.However, the use of such fluxes is not essential to the production fadherent coatings.

The invention is useful for the application of hot-dip coatings to serveas final protective coatings and also to serve as bonding means to unitethe chromium surfaces with other metals. Thus zinc coatings may serve asa base for solder bonds to other metals. A particularly suitablecombination involves the application of lowmelting' aluminum-siliconalloys to metallic aluminum sheaths.

In the application of the invention to the coating of metallic uranium,the chromium may be applied directly tothe uranium or over a lightcoating of another metal, such as nickel or copper, as described ingreater detail in my U. S. patent applications, Serial No. 572,093,entitled Nickel Coatings and Method of' Applying,'filed January 9, 1945,and Serial No. 571,673, entitled Copper Coatings and Method of.Applying, filed January 6, 1945.

The metallic uraniumpreferably is prepared for the application of thecoatings by an etchant treatment of the. metal surface with an aqueousetching solution containing chloride ions, i. e., a hydrochloric acidsolution or a molten hydrate of ferric chloride. Particularlysatisfactory treatments of this type involve anodic pickling of themetal in trichloracetic acid solution, for example an aqueous solutioncontaining 45% to 50% of trichloracetic acid, or in aqueous phosphoricacid solution containing about /z% of hydrochloric acid, to remove aone-half to one mil layer of the surface metal. Prior to the etchanttreatment metal may be cleaned by sandblasting, pumice-scrubbing,treating with organic solvents to remove grease or oily impurities, ortreating with a chemical solvent for the oxide film on the metal. Aque-011s nitric acid solutions containing from 30% to 55% HNO by Weight areespecially useful for this purpose. An additional short treatment withthis reagent should be applied after the etchant treatment in order toremove the coating of oxide or oxychloride from the etched. metalsurface.

The following examples further illustrate the inven tion. Unlessotherwise indicated, quantities are expressed in terms of weight.

Example 1 A metallic uranium rod is pickled in aqueous 15%trichloracetic acid solution at about 55 C. while a current is passedthrough the solution with the rod connected as anode for 10 minutes at acurrent density of 50 amperes per square foot. The rod is then rinsedwith cold water, dipped for four minutes in aqueous 35% HN 0 solution,rinsed again and electroplated in a chromic acid bath containing 400grams per liter of chromic acid at 50 C. and 250 amperes per sq. ft.current density until a chromium layer about 0.001" thick is formed(about 1 hour). A conforming lead anode spaced about one inch from therod is used. The freshly plated rod is then dipped into a molten zincbath containing 1% ofaluminum at 600 C. for approximately seconds. Therod is removed from the bath, shaken to free it from excess moltenmetal, placed on smooth Transite (asbestos cement) rollers and rolledslowly in air until the coating solidifies. A firmly adherentzinc-aluminum coating is obtained on the chromium-plated. rod.

Example 2 A rod plated and coated as in Example 1, upon removal. fromthe zinc bath, is spun at high speed for 5 seconds to remove excessmetal and is then dipped for 10 seconds into a molten aluminum-siliconalloy comprising 88 parts of aluminum and 12 parts of silicon at 638 C.The rod is withdrawn and cooled on rollers as in Example 1. A firmlyadherent coating of aluminum-silicon alloy over zinc is obtained on thechromium-plated uranium rod.

In this example the zinc bath has an important function of heating therod to a temperature approaching that of the aluminum-silicon bath andthus facilitating application of this alloy.

Example 3 A chromium-plated rod prepared as in Example 1 is dippedthrough a halide flux (prepared by first mixing 53 parts of potassiumchloride, 42 parts of lithium chloride and parts of sodium chloride andadding to the mixture parts of sodium fluoride) into a zinc bath of 99%zinc and 1% aluminum at a temperature of about 660 C. The rod is held inthis bath for 1 minute, then removed, spun to remove excess metal, anddipped for 4 seconds directly into an aluminum-silicon alloy containing88% aluminum and 12% silicon at 640 C. for 4 seconds. The rod is removedfrom this alloy and rolled until cool as in the preceding examples.

Example 4 A rod, chromium plated as in Example 1, is dipped through ahalide fiux of the same composition as used in Example 3 into a moltenzinc bath consisting of 95% zinc and 5% aluminum'at 650 C. The rod isheld in this bath for about 75 seconds, withdrawn, centrifuged for 5seconds, and passed through a flux of the same composition into an 88%aluminum 12% silicon alloy at 640 C. After 2 seconds in this bath therod is withdrawn and plunged into a preheated aluminum can containing asmall amount of aluminum-silicon at 600 C. Upon cooling the assembly thealuminum can is firmly united by the aluminum-silicon alloy and zinc tothe chromium-plated rod.

Example 5 A uranium rod, electroplated as in Example 1, is dipped for 30seconds into molten aluminum 2S at a temperature of about 750 C. Themolten aluminum is covered by a flux prepared by adding parts of NaF to100 parts of a 53% KCl, 42% LiCl, 5% NaCl mixture. The coating is cooledon rollers as in Example 1. A continuous aluminum coating on thechromium-plated rod is thus obtained.

Example 6 A machined cylindrical uranium rod about 8 inches long and 1/3 inches in diameter is prepared for electroplating by dipping intrichlorethene to remove grease, sand-blasting lightly, dipping inaqueous 35% I-INO at 30 C. for 4 minutes, rinsing with cold water,etching by anodic treatment in aqueous 50% trichloracetic acid solutionat between 50 C. and 60 C. for 10 minutes at 50 amperes per square footcurrent density, rinsing with cold water, dipping for 4 minutes in coldaqueous 35% HNO solution to clean the etched metal surface, and rinsingin cold water.

The cleaned, etched metal rod is immediately electroplated in a copperpyrophosphate electroplating bath, comprising 75 grams per liter ofcopper as pyrophosphate, at a current density of amperes per square footat 60 C. with air agitation until a copper plating about 0.0005 inchthick is formed (about minutes). The copper-plated rod is rinsed withwater and then electroplated in a chromic acid bath, containing 400grams per liter of chromic acid and 4 grams per liter of sulfuric acidat 50 C. and 250 amperes per sq. ft. current density until a chromiumlayer 0.0005 inch thick has formed (about 30 minutes). For the chromiumplating step a concentric lead anode comprising an expanded metalcylinder having a diameter about 2 inches greater than the rod is used.

The freshly chromium-plated rod is rinsed to remove electroplatingsolution, dried, and dipped directly into a sodium-modified 88% aluminum12% silicon alloy at 640 C. for 50 seconds. It is then removed from thealloy, shaken free of excess metal, and placed on smooth Transiterollers where it is rolled in contact with air at room temperature untilit has cooled.

Example 7 The process of the preceding example is repeated except thatthe copper electroplating step is conducted for only about 2 minutes toproduce a copper coating about 0.00003 inch thick and the uranium rodcovered with this flash coating is electroplated in a chromic acid bath,to a thickness of 0.0008 inch chromium and then hot dipped as in Example6. An equally adherent aluminum-silicon alloy coating affording adequateprotection for the uranium rod is obtained.

Example 8 A metallic uranium rod prepared for electroplating as inExample 6 is electroplated in a nickel sulfate electroplating bath,comprising 240 grams of NiSO .7H O, 45 grams of NiCl .6H O, and 30 gramsof H 30 at a current density of 25 amperesper sq. ft. at 40 C. until anickel plating about 0.0001 inch thick is formed (about 5 minutes). Theplated rod is then rinsed in water and electroplated in an aqueoussolution containing 400 grams per liter of chromic acid and 4 grams perliter of sulfuric acid at 50 C. until a chromium plating 0.0005 inchthick has been built up. A concentric lead anode comprising an expandedmetal cylinder having a diameter about 2 inches greater than the rod isused. The chromium-plated rod is rinsed, dried and dipped inaluminum-silicon as in Example 6. After rolling and cooling, a uniformadherent coating is obtained on the rod.

Example 9 The process of the preceding example is repeated except thatthe nickel electroplating step is conducted for only about 45 seconds toapply a nickel coating 0.000015 inch thick and the chromiumelectroplating step is conducted for about one hour to apply a chromiumplating 0.001 inch thick. As in the preceding example a firmly adherentcoating is obtained afiording adequate protection for the uranium rod.

In a similar manner hot-dip coatings of other aluminum alloys such ascopper-modified aluminum and magnesiumaluminum alloys may be applied tochromium platings on uranium and other metals. 7

It will be understood that I intend to include variations andmodifications of the invention and that the preceding examples areillustrations only and in no wise to be construed as limitations uponthe invention, the scope of Which is defined in the appended claims,wherein:

I claim:

1. The methodof protectively coating uranium, which comprises applying achromium plating to the uranium and bonding the chromium plating bymeans of molten aluminum-silicon alloy to an aluminum sheath.

2. The method of protectively coating uranium, which comprises applyinga flash coating of a metal of the group consisting of copper and nickel,applying a chromium plating thereover, and bonding the chromium platingby means of molten aluminum-silicon alloy to an aluminum sheath.

3. A uranium article having a chromium plating on a flash plating of thegroup consisting of nickel and copper and bonded to an aluminum sheathby means of an aluminum-silicon alloy containing about 12% of silicon.

a molten metal of the group consisting of aluminum andaluminum-containing alloys by passing said chromiumplated article thru amolten halide flux containing from 10 to 15 parts of NaF added to partsof a 53% KCl,"

42% LiCl, 5% NaCl mixture into a bath of said molten metal.

5. The method of protectively coating a uranium article which comprisesapplying a chromium plating thereto by electrolysis from a chromic acidbath, passing said chromium plated article thru a molten halide fluxcontaining from to parts of NaF added to 100 parts of a 53% KCl, 42%LiCl, 5% NaCl mixture into a molten metal bath consisting of 99% to zincand 1% to 5% aluminum, and then applying to the surface of said coatedarticle molten aluminum containing a minor proportion of silicon bypassing said coated article thru a molten halide flux containing from 10to 15 parts of NaF added to parts of a 53% KCl, 42% LiCl, 5% NaClmixture into a molten bath'of an 88% aluminum 12% silicon alloy.

6. A uranium article having a chromium plating over which is firmlybonded a metal coating of the group consisting of aluminum andaluminum-containing alloys.

7. A uranium article having a chromium plating over which is firmlybonded an adherent metal coating consisting of an alloy of aluminum witha minor proportion of silicon.

81 A uranium article having a chromium plating over which there is abonding layer consisting of an aluminumsilicon alloy containing about12% of silicon and an aluminum sheath, said bonding layer lying betweensaid chromium plating and said aluminum sheath.

References Cited in the file of this patent UNITED STATES PATENTS1,128,059 Schoop Feb. 9, 1915 1,607,676 Jirotka Nov. 23, 1926 1,764,132Wehr et al June 17, 1930 2,048,552 Johnson July 21, 1936 2,075,477 SmithMar. 30, 1937 FOREIGN PATENTS 93,967 Austria Aug. 25, 1923 305,544 GreatBritain Feb. 6, 1929

1. THE METHOD OF PROTECTIVELY COATING URANIUM, WHICH COMPRISES APPLYINGA CHROMIUM PLATING TO THE URANIUM AND BONDING THE CHROMIUM PLATING BYMEANS OF MOLTEN ALUMIUM-SILICON ALLOY TO AN ALUMINUM SHEATH.
 6. AURANIUM ARTICLE HAVING A CHROMIUM PLATING OVER WHICH IS FIRMLY BONDED AMETAL COATING OF THE GROUP CONSISTING OF ALUMIMUM ANDALUMINUM-CONTAINING ALLOYS.